When overridden in a derived class, the property gets a value that indicates whether the message is encrypted or encapsulated in a format that the EmailMessage class cannot parse.
Assembly: Microsoft.Exchange.Data.Transport (in Microsoft.Exchange.Data.Transport.dll)
Opaque messages include messages that are encrypted and some types of signed messages. When the property is true, the EmailMessage class provides read-only access to the message.
If you add or remove attachments or bodies from a signed e-mail message, the signature will be removed from the message. To retain the digital signature, we recommend that you create a new message that wraps the signed message by adding the signed message as an attachment to the new message.